High turn down low nox burner

ABSTRACT

An air-fuel burner includes a heat-transfer tube, an air-fuel mixing chamber, and an air-fuel nozzle. The air-fuel nozzle is coupled to the air-fuel chamber to communicate a combustible air-fuel mixture into a combustion chamber defined between the air-fuel nozzle and the heat-transfer tube. The combustible air-fuel mixture, when ignited, establishes a flame in the combustion chamber to produce heat which is transferred through heat-transfer tube to an adjacent medium external to the heat-transfer tube.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation-in-part of U.S. patentapplication Ser. No. 12/569,189, filed Sep. 29, 2009, entitled LOWNO_(x) INDIRECT FIRE BURNER, the disclosure of which is incorporatedherein by reference in its entirety.

BACKGROUND

The present disclosure relates to burners and particularly to indirectfire burners. More particularly, the present disclosure relates to anindirect fire air-fuel burner configured to produce low NO_(x)emissions.

SUMMARY

An air-fuel burner in accordance with the present disclosure comprisesan air-fuel nozzle adapted to receive a combustible air-fuel mixture.The air-fuel nozzle is configured to discharge the combustible air-fuelmixture into a combustion chamber. The discharged combustible air-fuelmixture is ignited to produce a flame in the combustion chamber.

In illustrative embodiments, the air-fuel nozzle is configured toprovide means for forming three nozzle exits to cause three separateflames to be established in the combustion chamber when the combustibleair-fuel mixture is ignited. In an illustrative embodiment, the firstnozzle exit is formed near an inner end of the elongated air-fuelnozzle, the third nozzle exit is formed at an opposite outer end of theelongated air-fuel nozzle, and the second (and largest) nozzle exit isformed near the opposite outer end and arranged to lie between the firstand third nozzle exits. Each nozzle exit is defined by one or morenozzle apertures opening into an air-fuel transfer passageway formed inthe air-fuel nozzle. The three nozzle exits are arranged in the air-fuelnozzle to cooperate to provide means for minimizing NO_(x) formationwithin the flames while maximizing flame temperature and operatingefficiency of the air-fuel burner.

In illustrative embodiments, the air-fuel burner comprises aheat-transfer tube, an air-fuel mixing chamber coupled to an upstreamend of the heat-transfer tube, and the air-fuel nozzle. The air-fuelnozzle is coupled in fluid communication to the air-fuel mixing chamberand is arranged to extend into an interior region formed within theheat-transfer tube. The air-fuel nozzle lies in an interior region ofthe heat-transfer tube and cooperates with the heat-transfer tube toform the combustion chamber there between. The air-fuel mixing chambermixes air and fuel to produce a combustible air-fuel mixture that iscommunicated in a downstream direction through the air-fuel nozzle anddischarged from the air-fuel nozzle to feed a flame formed in thecombustion chamber. The flame produces heat which heats theheat-transfer tube and is transferred from the heat-transfer tube to anadjacent medium outside the heat-transfer tube so that a temperature ofthe adjacent medium is raised.

In illustrative embodiments, about 10% to about 20% of the combustibleair-fuel mixture flowing through the air-fuel transfer passageway movesinto the combustion chamber through the first nozzle exit formed in theair-fuel nozzle. The first nozzle exit is configured to discharge acombustible air-fuel mixture that, when ignited, establishes a detachedfirst flame extending in radially outward directions from the air-fuelnozzle toward the heat-transfer tube. The detached first flame includesa root that is detached from the air-fuel nozzle and a tip that isarranged to stabilize on an interior surface of the heat-transfer tubeduring combustion.

In illustrative embodiments, about 40% to about 80% of the combustibleair-fuel mixture flowing through the air-fuel transfer passageway movesinto the combustion chamber through a second nozzle exit formed in theair-fuel nozzle. The second nozzle exit is arranged to lie inspaced-apart relation to the first nozzle exit in the downstreamdirection. The second nozzle exit is configured to discharge acombustible air-fuel mixture that is configured to improve burnerturn-down, whereby the operating range of the burner between a lowfiring rate and a high firing rate is improved through configuration ofthe second nozzle as a band of perforations. Upon igniting thecombustible air-fuel mixture exiting the second nozzle at a low firingrate, the second flame is created, that is attached to the secondnozzle, which extends in a radially outward direction from the air-fuelnozzle towards the heat-transfer tube. When the combustible air-fuelmixture exits the second nozzle at a high firing rate, the second flamedetaches from the second nozzle and extends in a radially outwarddirection from the air-fuel nozzle towards the heat-transfer tube. Thesecond flame, when it is detached, includes a root that is detached fromthe air-fuel nozzle and a tip that is arranged to stabilize on theinterior surface of the heat-transfer tube.

In illustrative embodiments, about 10% to about 20% of the combustibleair-fuel mixture flowing through the air-fuel transfer passageway movesinto the combustion chamber through a third nozzle exit formed in theair-fuel nozzle. The third nozzle exit is arranged to locate the secondnozzle exit between the first and third nozzle exits. The third nozzleexit is configured to discharge a combustible air-fuel mixture that,when ignited, establishes an attached third flame extending in thedownstream direction away from the air-fuel nozzle and the detachedfirst and second flames. The attached third flame includes a root thatis stabilized on a free end of the air-fuel nozzle and a tip thatextends freely in the downstream direction.

In illustrative embodiments, the air-fuel burner is configured in amanner that facilitates separation of the second flame produced from thesecond nozzle exit which is arranged to surround a circumference of theair-fuel nozzle and configured as a band of perforations positionedcircumferentially around the downstream end of the air-fuel transferconduit to create a circumferential second flame portion. Duringoperation, the first and second nozzle exits provide a means forcommunicating combustion products of the detached first flame and thesecond flame away from the air-fuel mixing chamber in the downstreamdirection through an upstream region in the combustion chamber inhabitedby the second flame when the combustible air-fuel mixture is dischargedfrom the second nozzle at a high firing rate (without being burned inthe detached second flame) and into a downstream region in thecombustion chamber inhabited by the attached third flame (to be burnedin the attached third flame).

Additional features of the present disclosure will become apparent tothose skilled in the art upon consideration of illustrative embodimentsexemplifying the best mode of carrying out the disclosure as presentlyperceived.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description particularly refers to the accompanying figuresin which:

FIG. 1 is a diagrammatic view of an air-fuel burner in accordance withthe present disclosure, showing that the air-fuel burner includes anair-fuel nozzle coupled to an air-fuel mixing chamber and is configuredto discharge a combustible air-fuel mixture (1) through a first nozzleexit to establish a first flame that detaches from the nozzle at a highfiring rate and extends in radially outward directions from the air-fuelnozzle that is stabilized on a liquid cooled low-temperature surface,(2) through a downstream second nozzle exit to establish a detachedsecond flame extending in radially outward directions from the air-fuelnozzle that is stabilized on the liquid cooled low-temperature surface,and (3) through a further downstream third nozzle exit to establish anattached third flame attached to and stabilized on the air-fuel nozzleand extending in the downstream direction away from the air-fuel nozzleand suggesting that combustion products of the detached first flame andthe second flame are drawn into the detached first flame and thatcombustion products of the detached first flame are drawn into thesecond flame so that the formation of NO_(x) is minimized duringcombustion, and that a portion of the combined products of combustionfrom the detached first flame and the second flame that are not drawninto the detached first flame and second flame during combustion movesdownstream to reach and be burned in the attached third flame;

FIG. 2 is a side elevation view of an illustrative air-fuel burner inaccordance with FIG. 1, with portions broken away, to reveal that theair-fuel burner includes an air-fuel nozzle arranged to lie within in aheat-transfer tube and that the air-fuel nozzle is coupled to anair-fuel mixing chamber wherein air from an air supply and fuel from afuel supply are mixed together to establish a combustible air-fuelmixture which moves downstream through an air-fuel transfer passagewayformed in the air-fuel nozzle and out of three nozzle exits formed inthe air-fuel nozzle to establish, when ignited, the first, second, andthird flames;

FIG. 3 is a partial perspective view of the air-fuel nozzle of FIGS. 1and 2 showing that the air-fuel nozzle includes an air-fuel transferconduit coupled at a first end to the air-fuel mixing chamber and thatthe air-fuel transfer conduit is formed to include a set of air-fueldischarge slots exposed to a combustible air-fuel mixture flowing in theair-fuel transfer passageway spaced-apart around the circumference ofthe air-fuel transfer conduit to establish the first nozzle exitassociated with the detached first flame as shown in FIG. 1 and a bandof perforations configured around the circumference and proximate asecond end of the air-fuel transfer conduit to define a circumferentialdischarge port exposed to a combustible air-fuel mixture flowing in theair-fuel transfer passageway of the air-fuel transfer conduit toestablish the second nozzle exit associated with the second flame,illustrated under conditions whereby the combustible air-fuel mixturehas a high firing rate, causing the second flame to detach from theexterior of the air-fuel transfer conduit as shown in FIG. 1 and showingthat an air-fuel discharge plate, attached to the second end of theair-fuel transfer conduit is formed to include a set of staged air-fueldischarge apertures communicating with a combustible air-fuel mixtureflowing in the air-fuel transfer passageway and opening in thedownstream direction to establish the third nozzle exit associated withthe attached third flame as shown in FIG. 1;

FIG. 4 is a sectional view of the air-fuel nozzle taken along line 4-4of FIG. 1 showing that the air-fuel burner is in a high-fire state,wherein the root of detached second flame is spaced-apart from theair-fuel nozzle and a tip of detached second flame is stabilized on theliquid cooled low-temperature surface, and showing that the second flamecomprises one large contiguous flame that surrounds the air-fuel nozzle;

FIG. 5 is view similar to FIG. 4 taken along line 4-4 showing thedetached second flame when the air-fuel burner is in a low-fire state,illustrating second flame stabilization and attachment to the surface ofthe air-fuel transfer conduit of the air-fuel nozzle, and showing thatthe second flame comprises one large contiguous flame surrounding andattached to the air-fuel nozzle; and

FIG. 6 is a partial perspective view showing a water heater including anair-fuel nozzle coupled in fluid communication to a source of acombustible air-fuel mixture and showing that the air-fuel nozzle isarranged to lie within an interior region of a heat-transfer tube toproduce three flames that generate heat which heats the heat-transfertube and transfers from the heat-transfer tube into water flowingthrough a water-heating chamber formed between a water vessel and theheat-transfer tube so that a temperature of water adjacent to theheat-transfer tube is raised.

DETAILED DESCRIPTION

An illustrative air-fuel burner 10, in accordance with the presentdisclosure, includes a heat-transfer tube 12, an air-fuel mixing chamber14, and an air-fuel nozzle 16 as shown in FIG. 1. Air-fuel nozzle 16 iscoupled in fluid communication to air-fuel mixing chamber 14 and isarranged to extend into an interior region 80 of heat-transfer tube 12as shown in FIG. 2. Air-fuel mixing chamber 14 mixes air 20 from an airsupply 22 and fuel 24 from a fuel supply 26 to establish a combustibleair-fuel mixture 28. Combustible air-fuel mixture 28 flows throughair-fuel nozzle 16 into a combustion chamber 30 defined betweenheat-transfer tube 12 and air-fuel nozzle 16 and is ignited to form aflame. The flame generates heat that heats heat-transfer tube 12 so thatheat is transferred from heat-transfer tube 12 to an adjacent medium 13of any suitable kind as suggested in FIG. 1.

As shown in FIG. 1, air-fuel nozzle 16 provides means for forming afirst nozzle exit 31, a second nozzle exit 32, and a third nozzle exit33 that communicate combustible air-fuel mixture 28 from air-fueltransfer passageway 39 formed in air-fuel nozzle 16 into combustionchamber 30. First nozzle exit 31 is formed in air-fuel nozzle 16 andcommunicates combustible air-fuel mixture 28 to establish, when aportion of combustible air-fuel mixture 28 is ignited, a detached firstflame 41 extending in radially outward directions 34 in combustionchamber 30 from air-fuel nozzle 16 toward heat-transfer tube 12.Detached first flame 41 is stabilized on an interior surface 36 ofheat-transfer tube 12 in an illustrative embodiment as suggested in FIG.1.

Second nozzle exit 32, as suggested in FIG. 1, is formed in air-fuelnozzle 16 and is arranged to lie in spaced-apart relation to firstnozzle exit 31 in a downstream direction 38 away from air-fuel mixingchamber 14. Second nozzle exit 32, configured to improve burnerturn-down, whereby the operating range of the burner between a lowfiring rate and a high firing rate is improved, communicates combustibleair-fuel mixture 28 into combustion chamber 30 to establish, when aportion of combustible air-fuel mixture 28, exiting the second nozzleexit 32 at a high firing rate is ignited, the second flame 42 detachesfrom the air-fuel nozzle and extends in radially outward directions 34from air-fuel nozzle 16 toward heat-transfer tube 12. Detached secondflame 42 is stabilized on interior surface 36 of heat-transfer tube 12is an illustrative embodiment as suggested in FIG. 1. Second nozzle exit32 is configured in a manner that facilitates stabilization andattachment of second flame 42 (shown in FIG. 5) to the exterior of theair-fuel nozzle 16 of air-fuel transfer conduit 40 when combustibleair-fuel mixture 28 has a low firing rate. Upon increasing the firingrate of the combustible air-fuel mixture 28, second flame 42 detachesfrom the exterior of the air-fuel nozzle 16 of air-fuel transfer conduit40 as illustrated in FIG. 1 and FIG. 4.

As shown in FIG. 1, third nozzle exit 33 is formed in air-fuel nozzle 16and is arranged to lie in spaced-apart relation to second nozzle exit 32in downstream direction 38 to locate second nozzle exit 32 between firstand third nozzle exits 31, 33. Third nozzle exit 33 communicatescombustible air-fuel mixture 28 into combustion chamber 30 to establish,when a portion of combustible air-fuel mixture 28 is ignited, anattached third flame 43 extending in downstream direction 38 away fromair-fuel nozzle 16. Attached third flame 43 is stabilized on air-fuelnozzle 16, in an illustrative embodiment as suggested in FIG. 1.

Illustratively, air-fuel nozzle 16 includes an air-fuel transfer conduit40, and an air-fuel discharge plate 44 as shown in FIG. 3. Air-fueltransfer conduit 40 is formed to include air-fuel transfer passageway 39and is coupled in fluid communication to air-fuel mixing chamber 14 toreceive an air-fuel mixture discharged from air-fuel mixing chamber 14.

As shown in FIG. 2, air-fuel transfer conduit 40 includes an upstreamend 48 and a downstream end 50 arranged to lie in spaced-apart relationin downstream direction 38 opposite to upstream end 48. Air-fueltransfer conduit 40 is further formed to include an air-fuel transferpassageway 39 communicating combustible air-fuel mixture 28 fromair-fuel mixing chamber 14 between upstream end 48 and downstream end 50as shown in FIG. 2. Air-fuel transfer conduit 40 is coupled to air-fuelmixing chamber 14 at upstream end 48 and coupled to discharge plate 44at the air-fuel transfer conduit 40 downstream end 50.

As shown in FIGS. 2 and 3, first nozzle exit 31 and second nozzle exit32 are formed in air-fuel transfer conduit 40. Illustratively, firstnozzle exit 31 is arranged to lie in spaced-apart relation to air-fuelmixing chamber 14 in downstream direction 38. Second nozzle exit 32 isarranged to lie in spaced-apart relation to first nozzle exit 31 indownstream direction 38 at downstream end 50 of air-fuel transferconduit 40.

First nozzle exit 31 is defined by a series of air-fuel discharge slots52 arranged to lie in spaced-apart relation to one another around acircumference 54 of air-fuel transfer conduit 40 as shown in FIG. 3.Illustratively, series of air-fuel discharge slots 52 is defined byfirst, second, third, fourth, fifth, and sixth air-fuel discharge slots52 a, 52 b, 52 c, 52 d, 52 e, and 52 f that are positioned to lie ingenerally equally spaced-apart relation to one another.

Second nozzle exit 32 illustratively is defined by a band ofperforations 56 positioned around the circumference 54 of the downstreamend of air-fuel transfer conduit 40 as shown in FIG. 3. Illustratively,the band of perforations 56, having a width W2, function as a largeair-fuel discharge port, which, at low firing, the flame created uponigniting the air fuel mixture attaches to the exterior surface area ofthe air-fuel transfer conduit 40 into which the band of perforations 56are configured. Configuring the second nozzle exit 32 as a band ofperforations allows for increased turn down and maintenance of flamestability while preventing the flame from flashing back inside theair-fuel transfer conduit.

As shown in FIG. 3, the air fuel nozzle 16 is comprised of an air-fueltransfer conduit 40 that is directly coupled at its down stream end 50to an air fuel discharge plate 40. The downstream end 50 of the air-fueltransfer conduit 40 is configured with a band of perforations 56 thereinthat function as a large air fuel discharge port that generates aflaming circumferential band that communicates combined combustionproducts 74 of detached first and second flames 41, 42 away fromair-fuel mixing chamber 14 in downstream direction 38 through anupstream region 98 in combustion chamber 30 inhabited by detached secondflame 42 without being burned in second flame 42 and into a downstreamregion 100 in combustion chamber 30 inhabited by attached third flame 43to reach and be burned in third flame 43 as suggested in FIG. 1.

Combustible air-fuel mixture 28 moves downstream through air-fueltransfer passageway 39 formed in air-fuel transfer conduit 40 and isturned in radially outward directions 34 by air-fuel discharge plate 44.Combustible air-fuel mixture 28 moves through the band of perforations56, which function as a large air fuel discharge port, configured togenerate a flaming circumferential band as illustrated in FIGS. 4 and 5.Wherein, as illustrated in FIG. 4, when the air-fuel burner is in ahigh-fire state, the root of second flame 42 is spaced-apart D2 from theair-fuel nozzle 16 and a tip of detached second flame 42T is stabilizedon the liquid cooled low-temperature surface 36, and showing that thesecond flame 42 comprises one large contiguous flame that surrounds theair-fuel nozzle 16. FIG. 5 illustrates when the air-fuel burner is in alow-fire state and the root of flame portions 42 that comprise thesecond flame 58 are attached to the to the air-fuel nozzle 16 creatingone large contiguous flame 58 surrounding and attached to the air-fuelnozzle 16.

Third nozzle exit 33, as shown in FIG. 3, is formed in air-fueldischarge plate 44. Third nozzle exit 33 is defined by an illustrativeseries of staged air-fuel discharge apertures 64 arranged to extend in apattern to lie between a center 66 and a perimeter edge 68 of air-fueldischarge plate 44 as shown in FIG. 3. Other patterns of staged air-fueldischarge apertures are possible and contemplated within the scope ofthe present disclosure. Attached third flame 43, when a portion ofcombustible air-fuel mixture 28 is ignited, extends between center 66and perimeter edge 68 to initiate and maintain ignition of detachedsecond flame 42.

In one embodiment of the present disclosure, first nozzle exit 31 isconfigured to communicate about 10% to about 20% of combustible air-fuelmixture 28 by volume into combustion chamber 30. Second nozzle exit 32is configured to communicate about 40% to about 80% of combustibleair-fuel mixture 28 by volume into combustion chamber 30. Third nozzleexit 33 is configured to communicate about 10% to about 20% ofcombustible air-fuel mixture 28 by volume in downstream direction 38.

As suggested in FIG. 1, about 10% to about 20% of combustible air-fuelmixture 28 by volume exits through first nozzle exit 31 to establishdetached first flame 41. As detached first flame 41 combusts, detachedfirst flame 41 forms first flame combustion products 71. A portion offirst flame combustion products 71 moves in an upstream direction 70opposite to downstream direction 38 toward air-fuel mixing chamber 14and first flame combustion products 71 are drawn into combustibleair-fuel mixture 28 exiting first nozzle exit 31. First flame combustionproducts 71 mix with combustible air-fuel mixture 28 exiting firstnozzle exit 31 and operate as an inert component during combustion tominimize thermal nitrous oxide (NO_(x)) formation in detached firstflame 41. Another portion of first flame combustion products 71 moves indownstream direction 38 to mix with combustible air-fuel mixture 28exiting second nozzle exit

Second nozzle exit 32 communicates about 40% to about 80% of combustibleair-fuel mixture 28 to combustion chamber 30. As detached second flame42 combusts, detached second flame 42 forms second flame combustionproducts 72. A first portion of second flame combustion products 72moves in downstream direction 38. Another portion of second flamecombustion products 72 moves in upstream direction 70 toward detachedfirst flame 41 and is drawn into combustible air-fuel mixture 28 exitingfirst nozzle exit 31 to minimize NO_(x) formation in detached firstflame 41. Similarly, a portion of first flame and second flamecombustion products 71, 72 are mixed with combustible air-fuel mixture28 exiting second nozzle exit 32 and operate as inert components duringcombustion of detached second flame 42 to minimize NO_(x) formation indetached second flame 42.

As suggested in FIG. 1, combined combustion products 74 of detachedfirst and second flames 41, 42 move in downstream direction 38 are notcompletely oxidized in detached second flame 42. Third flame 43 operatesto oxidize any unburned hydrocarbons in combined combustion products 74and to minimize carbon monoxide (CO) formed by detached first and secondflames 41, 42.

Illustratively, detached first flame 41 includes a root 41R and a tip41T as shown in FIG. 1. Root 41R is positioned to lie between air-fueltransfer conduit 40 and heat-transfer tube 12. Tip 41T is positioned tolie between root 41R and heat-transfer tube 12. As an example, root 41Ris spaced-apart from air-fuel transfer conduit 40 a first distance D1 asshown in FIG. 1. First distance D1 allows detached first and secondflame combustion products 71, 72 to be mixed into combustible air-fuelmixture 28 exiting first nozzle exit 31 prior to ignition of detachedfirst flame 41. Tip 41T of detached first flame 41 maintains combustionby extending out and stabilizing on interior surface 36 of heat-transfertube 12. As a result of root 41R being spaced-apart from first nozzleexit 31, the temperature of air-fuel transfer conduit 40 around firstnozzle exit 31 is minimized further minimizing NO_(x) formation fromdetached first flame 41.

Second flame portions 42, which detaches from the air transfer conduit40 upon increasing the firing rate of the combustible air-fuel mixture,when detached includes a root 42R and a tip 42T as shown in FIG. 1. Root42R is positioned to lie between air-fuel transfer conduit 40 andheat-transfer tube 12. Tip 42T is positioned to lie between root 42R andheat-transfer tube 12. As an example, root 42R is arranged to lie inspaced-apart relation to air-fuel transfer conduit 40 a relativelysmaller second distance D2 as shown in FIG. 1. Second distance D2 allowsdetached first and second flame combustion products 71, 72 to be mixedinto combustible air-fuel mixture 28 exiting second nozzle exit 32 priorto ignition of detached second flame 42. Detached second flame 42, likedetached first flame 41, maintains combustion by extending out and ontointerior surface 36 of heat-transfer tube 12 to stabilize on interiorsurface 36. As a result of root 42R being spaced-apart from secondnozzle exit 32, the temperature of air-fuel transfer conduit 40 aroundsecond nozzle exit 32 is minimized further minimizing NO_(x) formationfrom detached second flame 42. Upon decreasing the firing rate of thecombustible air-fuel mixture, the distance between the root of thesecond flame portions 42 and the air transfer conduit is decreased untilthe second flame portions 42 attach to the air-transfer conduit 40. Whenthe firing rate of the combustible air fuel mixture is low, root 42R isarranged to lie in contact with the air transfer conduit 40 and tip 42Tis positioned to lie between air transfer conduit 40 and heat transfertube 12.

Attached third flame 43 includes a root 43R and a tip 43T as shown inFIG. 1. Root 43R is arranged to lie on air-fuel discharge plate 44. Tip43T is arranged to lie in spaced-apart relation to root 43R and extendin downstream direction 38. Attached third flame 43 is stabilized duringcombustion on air-fuel discharge plate 44 by any suitable means ofattachment.

First and second nozzle exits 31, 32 are formed in air-fuel transferconduit 40 so that detached first and second flame combustion products71, 72 are mixed within combustible air-fuel mixture 28 flowing throughfirst and second nozzle exits 31, 32. Flame combustion products 71, 72are able to move within combustion chamber 30 as result of spacingbetween first and second nozzle exits 31, 32 being configured to blockthe merging of detached first and second flames 41, 42

As an example, a distance d1 is defined between first nozzle exit 31 andsecond nozzle exit 32. Distance d1 is a function of a diameter d2 ofair-fuel transfer conduit 40 as shown in FIG. 3. Illustratively,distance d1 is between about 1.8 and about 4.0 times diameter 84 ofair-fuel transfer conduit 40. Distance d1 permits detached first flame41 to ignite and stabilize on interior surface 36 of heat-transfer tube12 while permitting detached second flame 42 to ignite and stabilize oninterior surface 36. Distance d1 also operates to block detached firstand second flames 41, 42 from merging together to form one flame and tomaximize mixing of combustion products 71, 72 into detached first andsecond flames 41, 42.

As shown in FIG. 3, each of air-fuel discharge slots 52 are configuredto have a first width W1 defined between generally parallel sides 87, 89of air-fuel discharge slots 52. The band of perforations 56 positionedaround the circumference of the air-fuel transfer conduit performs as alarge circumferential discharge port that is configured to have arelatively larger second width W2 defined by the width of the band ofperforations 56. First width W1 is configured to be relatively smallerthan second width W2 so that the appropriate volumetric flow ofcombustible air-fuel mixture 28 is communicated through associatednozzle exits 31, 32.

Air-fuel nozzle 16 of air-fuel burner 10 is shown in a high-fire statein FIG. 4 and in a low-fire state in FIG. 5. The high-fire state ofair-fuel burner 10 is associated with maximized volumetric flow ofcombustible air-fuel mixture 28 through air-fuel transfer conduit 40 tomaximize heat production and as a consequence heat transfer throughheat-transfer tube 12. The low-fire state of air-fuel burner 10 isassociated with a volumetric flow that is lower than the maximizedvolumetric flow of combustible air-fuel mixture 28. The low-fire stateis used, as an example, during start-up of air-fuel burner 10 to warmthe system and minimize thermal shock. After warming is complete,high-fire state may be used or another volumetric flow amount that isbetween high-fire state and low-fire state depending on the amount ofheat needed to be transferred from heat-transfer tube 12 to adjacentmedium 13.

As shown in FIG. 4, air-fuel nozzle 16 is shown when air-fuel burner 10is in the high-fire state. The circumferential band of flames 58 extendsfrom second nozzle exit 32 in a radially outward direction 34 tostabilize on interior surface 36. Illustratively, the root of detachedsecond flame portions 42 is positioned to lie in spaced-apart relationto air-fuel transfer conduit 40 second distance D2 as shown in FIG. 4.During low-fire state, the root of second flame portions 42 attach toair-fuel transfer conduit 40 as shown in FIG. 5 as a result of the lowervolumetric flow of combustible air-fuel mixture 28.

As illustrated in FIG. 1, flames 41, 42, 43 are arranged to have varyingflame temperatures relative one another to minimize NO_(x) formation inflames 41, 42, 43. Detached first flame 41 is configured to have a firstflame temperature. Second flame 42 is configured to have a relativelylarger second flame temperature relative to detached first flame 41 as aresult of the higher volumetric flow of combustible air-fuel mixture 28.Attached third flame 43 is configured to have a relatively larger thirdflame temperature relative to detached first and second flames 41, 42.First and second flame temperatures are lower than third flametemperature as a result of detached first and second flames 41, 42quenching on interior surface 36 of heat-transfer tube 12, detachmentfrom air-fuel transfer conduit 40, and mixing of combined combustionproducts 74 into combustible air-fuel mixture 28 coming out of first andsecond nozzle exits 31, 32.

Air-fuel burner 10, as shown in FIG. 1, may be used in a boiler, afire-tube heater, a hot-water heater, a liquid-solution heater, or anyother suitable device. Illustratively, air-fuel burner 10 may be also beretrofitted onto an existing device to replace a less efficient air-fuelburner or a higher NO_(x) producing burner.

Heat-transfer tube 12 includes an interior surface 36 and an exteriorsurface 80 arranged to lie in spaced-apart relation to interior surface36 as shown in FIG. 2. First and second flames 41, 42 stabilize oninterior surface 36 during combustion. The temperature of heat-transfertube 12 in regions where detached first and second flames 41, 42stabilize is minimized by an adjacent medium 13 in contact with exteriorsurface 80 as shown in FIG. 1. Adjacent medium 13, illustratively water,absorbs the heat to cause NO_(x) formation from detached first andsecond flames 41, 42 to be further minimized. In other embodiments,adjacent medium 13 is glycol, a glycol-water mixture, or any othersuitable alternative.

As shown in FIG. 6, an illustrative water heater 200 includes air-fuelnozzle 16, heat-transfer tube 12, and a water vessel 202. Water vessel202 is coupled to heat-transfer tube 12 to define a water-heating cavity204 there between. Illustratively, cold water 206 flows intowater-heating cavity 204 through a cold-water inlet 206 and hot water208 flows out of water-heating cavity 204 through a hot-water outlet 210as suggested in FIG. 6. Illustratively, water heater 200 furtherincludes a water-heater shell 212 configured to enclose water vessel202, heat-transfer tube 12, and air-fuel nozzle 16. Water-heater shell212 cooperates with water vessel 202 and heat-transfer tube 12 to definea combustion-products passageway 214 there between. Illustratively, acombustion-product outlet 216 is formed in water-heater shell 212 toallow combined combustion products 218 to escape water heater 200 assuggested in FIG. 6.

Water heater 200 further includes a combustible air-fuel mixture source220 which is coupled in fluid communication to air-fuel nozzle 16 toprovide combustible air-fuel mixture 28 to air-fuel nozzle 16. Asdiscussed previously, combustible air-fuel mixture 28 flows throughfirst, second, and third nozzle exits 31, 32, 33 formed in air-fuelnozzle to form detached first and second flames 41, 42 and attachedflame 41 when ignited. As shown in FIG. 7, detached first and secondflames 41, 42 from air-fuel nozzle 16 toward heat-transfer tube 12 tostabilize thereon. Illustratively, water 222 within water vessel 202operates to cool heat-transfer tube 12 to aid in minimizing NO_(x)formation associated with first, second, and third flames 41, 42, 43.

Air-fuel burner 10 is configured to provide minimized NO_(x) emissionsand maximized efficiency in indirect fired applications such as boilersand fire-tube heaters. NO_(x) is controlled in air-fuel burner 10 inaccordance with the present disclosure by positioning first, second, andthird flames 41, 42, 43, recirculation combined combustion products 74into first and second flames 41, 42, flame stabilization onheat-transfer tube 12, and cooling of interior surface 36 ofheat-transfer tube 12 by adjacent medium 13.

During operation of air-fuel burner 10, attached third flame 43, ignitedoriginally with igniter 76 operates as an ignition sources for detachedsecond flame 42. Attached third flame 43 has a small (about 10% to about20%) volumetric fraction of combustible air-fuel mixture 28 emitted fromair-fuel nozzle 16. Attached third flame 43 is stabilized, for example,on air-fuel discharge plate 44. It is within the scope of thisdisclosure to stabilize third flame 42 in any suitable manner. Secondflame 42 which has a relatively larger (about 40% to about 80%)volumetric fraction of combustible air-fuel mixture 28 emitted fromair-fuel nozzle 16, detaches from the air transfer conduit uponincreasing the firing rate of the combustible air-fuel mixture, whichwhen detached is suspended around air-fuel discharge plate 44 andpropagates freely between air-fuel discharge plate 44 and interiorsurface 36 of heat-transfer tube 12. As an example, detached first flame41 has a relatively smaller (about 10% to about 20%) volumetric fractionof combustible air-fuel mixture 28 exiting through first nozzle exit 31that mixes with second flame combustion products 72 to the point wherefirst flame 41 is not self sustaining and burns as flameless combustionwhich is relatively transparent.

Illustratively, first flame 41 does not have any attachment mechanismsas a result of the exit velocity of combustible air-fuel mixture 28exiting through associated first nozzle exit 31 being higher than theflame propagation speed. Minimizing flame attachment points causes flameretention hot spots and eddy dwell time to be minimized. Detached firstflame 41 is spaced-apart from second flame 42 so that detached firstflame 41 forms its own independent flame separate from second flame 42.Detached first flame 41 operates to produce first flame combustionproducts 71, which move in downstream direction 38 to mix into secondflame 42. Second flame 42 has no retention mechanism and propagatesfreely between air-fuel transfer conduit 40 and interior surface 36 ofheat-transfer tube 12.

First and second flames 41, 42 are illustratively configured to besmoother flow. Turbulent flow of combustible air-fuel mixture 28 shouldbe minimized when exiting first and second nozzle exits 31, 32 so thatflame lift-off is promoted. As an example, first and second flames 41,42 are configured to be non-symmetrical or uneven when viewed about theline 4-4 of FIG. 1. The imbalance in first and second flames 41, 42encourages a self-induced internal recirculation of combined combustionproducts 74 from first and second flames 41, 42 into first and secondflames 41, 42.

As shown in FIGS. 1 and 2, air-fuel mixing chamber 14 operates toprovide a homogeneous mixture of air 20 and fuel 24 to establishcombustible air-fuel mixture 28. Within air-fuel mixing chamber 14, air20 and fuel 24 are converted into turbulent flows which promoteefficient mixing to form a turbulent flow of combustible air-fuelmixture 28 into air-fuel transfer passageway 39 formed in air-fueltransfer conduit 40. Air-fuel transfer conduit 40 is configured to havea length sufficient to allow the turbulent flow of combustible air-fuelmixture 28 to return to a smoother flow within air-fuel transfer conduit40. Smoother flow within air-fuel transfer conduit 40 allows forsmoother flow out of first, second, and third nozzle exits 31, 32, 33 tooccur.

Illustratively, air-fuel burner 10 is configured to provide less thanabout 10 ppm of NO_(x) when using about 15% to about 30% excess air.Air-fuel burner 10, as an example, may use about 30% excess air or lesswithout the use of any external combustion product recirculation. Inaddition, air-fuel burner 10 may operate between about 2% and about 8%Oxygen (O.sub.2) and achieve about a 6 to 1 emission and thermalturndown ratio.

1. An air-fuel burner comprising a heat-transfer tube formed to includean interior region and adapted to discharge heat to an adjacent mediumlocated outside the heat-transfer tube when exposed to heat from a flamegenerated in the interior region, an air-fuel mixing chamber adapted tomix air from an air supply and fuel from a fuel supply to establish acombustible air-fuel mixture therein, and an air-fuel nozzle coupled tothe air-fuel mixing chamber and arranged to extend into the interiorregion of the heat-transfer tube, the air-fuel nozzle being configuredto provide means for forming three nozzle exits communicating with acombustion chamber defined in the interior region and located betweenthe air-fuel nozzle and the heat-transfer tube to cause the combustibleair-fuel mixture to exit from the air-fuel nozzle into the combustionchamber through a first nozzle exit formed in the air-fuel nozzle toestablish, when a portion of the combustible air-fuel mixture flowingthrough the first nozzle exit is ignited, a detached first flameextending in radially outward directions in the combustion chamber fromthe air-fuel nozzle toward the heat-transfer tube, and the detachedfirst flame includes a root positioned to lie between the air-fuelnozzle and the heat-transfer tube and a tip arranged to stabilize on aninterior surface of the heat-transfer tube, a second nozzle exit formedin the air-fuel nozzle and arranged to lie in spaced-apart relation tothe first nozzle exit in a downstream direction away from the air-fuelmixing chamber to establish, when a portion of the combustible air-fuelmixture flowing through the second nozzle exit is ignited, a secondflame extending in radially outward directions in the combustion chamberfrom the air-fuel nozzle toward the interior surface of theheat-transfer tube, and the second flame includes a root positioned tolie between the air-fuel nozzle and the heat-transfer tube and a tiparranged to stabilize on the interior surface of the heat-transfer tubewhen the firing rate of the combustible air fuel mixture is high,wherein the second flame is attached to the air-fuel nozzle when thefiring rate of the combustible air fuel mixture is low and detaches fromthe air fuel nozzle when the firing rate of the combustible air fuelmixture is increased to exceed a threshold firing rate, and a thirdnozzle exit formed in the air-fuel nozzle and arranged to lie inspaced-apart relation to the second nozzle exit in the downstreamdirection to locate the second nozzle exit between the first and thirdnozzle exits and to establish, when a portion of the combustibleair-fuel mixture flowing through the third nozzle exit is ignited, anattached third flame extending in the downstream direction away from theair-fuel nozzle and the detached first and second flames, and theattached third flame includes a root stabilized on the air-fuel nozzleand a tip extending in the downstream direction.
 2. The air-fuel burnerof claim 1, wherein the air-fuel nozzle includes an air-fuel transferconduit and an air-fuel discharge plate, the air-fuel transfer conduithas an upstream end and a downstream end arranged to lie in spaced-apartrelation opposite the upstream end and the air-fuel transfer conduit iscoupled to the air-fuel mixing chamber at the upstream end and to theair-fuel discharge plate at the downstream end.
 3. The air-fuel burnerof claim 2, wherein the first nozzle exit is defined by a series ofair-fuel discharge slots formed in the air-fuel transfer conduit andarranged to lie in circumferentially spaced-apart relation to oneanother around a circumference of the air-fuel transfer conduit.
 4. Theair-fuel burner of claim 3, wherein the second nozzle exit is defined bya band of perforations formed in the air-fuel transfer conduit andarranged to lie circumferentially around the circumference of theair-fuel transfer conduit.
 5. The air-fuel burner of claim 4, whereinthe third nozzle exit is defined by a series of staged air-fueldischarge apertures formed in the air-fuel discharge plate and arrangedto extend in a pattern between a center of the air-fuel discharge plateand a perimeter edge of the air-fuel discharge plate to cause theattached third flame, when ignited, to extend between the center and theperimeter edge to maintain ignition of the detached second flame.
 6. Theair-fuel burner of claim 1, wherein the first nozzle exit is configuredto provide means for communicating about 10% to about 20% of thecombustible air-fuel mixture, the second nozzle exit is configured toprovide means for communicating about 40% to about 80% of thecombustible air-fuel mixture, and the third nozzle exit is configured toprovide means for communicating about 10% to about 20% of thecombustible air-fuel mixture by volume through the air-fuel nozzle. 7.The air-fuel burner of claim 1, wherein a distance d1 between the firstnozzle exit and the second nozzle exit is between about 1.8 and about 4times a diameter d2 of the air-fuel nozzle.
 8. The air-fuel burner ofclaim 1, wherein the root of the detached first flame is positioned tolie in spaced-apart relation to the air-fuel nozzle a first distance D1and the root of the detached second flame is positioned to lie inspaced-apart relation to the air-fuel nozzle a relatively smaller seconddistance D2.
 9. The air-fuel burner of claim 1, wherein the air-fuelnozzle includes an air-fuel transfer conduit and an air-fuel dischargeplate, the air-fuel transfer conduit has an upstream end and adownstream end arranged to lie in spaced-apart relation opposite to theupstream end and the air-fuel transfer conduit is coupled to theair-fuel mixing chamber at the upstream end and to the air-fueldischarge plate at the downstream end, and wherein the first nozzle exitis defined by a series of air-fuel discharge slots formed in theair-fuel transfer conduit and arranged to lie in circumferentiallyspaced-apart relation to one another around a circumference of theair-fuel transfer conduit, the second nozzle exit is defined by a bandof perforations positioned circumferentially around the downstream endof the air-fuel transfer conduit.
 10. The air-fuel burner of claim 9,wherein the series of air-fuel discharge slots is defined by a firstdischarge slot, a second discharge slot, a third discharge slot, afourth discharge slot, a fifth discharge slot, and a sixth dischargeslot and each discharge slot is positioned to lie in spaced-apartrelation equally to one another around the circumference of the air-fueltransfer conduit from one another.
 11. The air-fuel burner of claim 4,wherein the second nozzle exit provides a means to improve operatingrange of the burner between a low firing rate and a high firing rate,wherein at low firing rate, the second flame exiting from the secondnozzle exit stabilizes and attaches to a surface of the air-fuel nozzleinto which the second nozzle exit is formed.
 12. An air-fuel burnercomprising a heat-transfer tube formed to include an interior region, anair-fuel mixing chamber configured to establish a combustible air-fuelmixture therein, and an air-fuel nozzle coupled to the air-fuel mixingchamber and arranged to extend into the interior region of theheat-transfer tube, the air-fuel nozzle formed to include three nozzleexits communicating with a combustion chamber defined in the interiorregion between the air-fuel nozzle and the heat-transfer tube to movethe combustible air-fuel mixture from the air-fuel nozzle into thecombustion chamber through a first nozzle exit formed in the air-fuelnozzle to establish, when a portion of the combustible air-fuel mixtureflowing through the first nozzle exit is ignited, a detached first flameextending in radially outward directions in the combustion chamber fromthe air-fuel nozzle toward the heat-transfer tube, and the detachedfirst flame includes a root positioned to lie between the air-fuelnozzle and the heat-transfer tube and a tip arranged to stabilize on aninterior surface of the heat-transfer tube, a second nozzle exit formedin the air-fuel nozzle and arranged to lie in spaced-apart relation tothe first nozzle exit in a downstream direction away from the air-fuelmixing chamber to establish, when a portion of the combustible air-fuelmixture flowing the through the second nozzle exit is ignited, a secondflame attached to and extending in radially outward directions from theair fuel nozzle into the combustion chamber toward the interior surfaceof the heat-transfer tube, wherein the attached second flame isstabilized on the air fuel nozzle within the heat-transfer tube, and athird nozzle exit formed in the air-fuel nozzle and arranged to lie inspaced-apart relation to the second nozzle exit in the downstreamdirection to locate the second nozzle exit between the first and thirdnozzle exits and to establish, when a portion of the combustibleair-fuel mixture flowing through the third nozzle exit is ignited, aattached third flame extending in the downstream direction away from theair-fuel nozzle and the detached first and second flames, and theattached third flame includes a root stabilized on the air-fuel nozzleand a tip extending in the downstream direction.
 13. The air-fuel burnerof claim 12, wherein the second flame is attached to the air-fuel nozzlewhen the firing rate of the combustible air fuel mixture is low.
 14. Theair-fuel burner of claim 13, wherein the second flame detaches from theair fuel nozzle when the firing rate of the combustible air fuel mixtureis increased to exceed a threshold firing rate.
 15. The air-fuel burnerof claim 14, wherein the air-fuel nozzle includes an air-fuel transferconduit and an air-fuel discharge plate, the air-fuel transfer conduithas an upstream end and a downstream end arranged to lie in spaced-apartrelation opposite the upstream end and the air-fuel transfer conduit iscoupled to the air-fuel mixing chamber at the upstream end and to theair-fuel discharge plate at the downstream end.
 16. An air-fuel burnercomprising an elongated air-fuel nozzle adapted to receive a combustibleair-fuel mixture and configured to provide means for forming threenozzle exits to cause three separate flames to be established in thecombustion chamber when the combustible air-fuel mixture is ignited andwherein the three nozzle exits are defined by a first nozzle exit formedin the elongated air-fuel nozzle and positioned to lie in spaced-apartrelation to an inner end of the elongated air-fuel nozzle, a relativelylarger second nozzle exit formed in the elongated air-fuel nozzle andpositioned to lie in spaced-apart relation to first nozzle exit near anopposite outer end of the elongated air-fuel nozzle, and a relativelysmaller third nozzle exit formed in the elongated air-fuel nozzle andpositioned to lie at the opposite outer end of the elongated air-fuelnozzle to locate the relatively larger second nozzle exit between firstnozzle exit and the relatively smaller third nozzle exit and the first,second, and third nozzle exits are arranged in the elongated air-fuelnozzle to cooperate to provide means for minimizing NO_(x) formationassociated with the three flames during combustion while maximizingoperating efficiency of the air-fuel burner and to provide a means forimproved burner turn-down, whereby the operating range of the burnerbetween a low firing rate and a high firing rate is improved through theconfiguration of the second nozzle exit that facilitates attachment toand stabilization there upon of a second flame exiting from the secondnozzle exit to a surface of the air-fuel nozzle into which the secondnozzle exit is formed.
 17. The air-fuel burner of claim 16, wherein theelongated air-fuel nozzle includes an air-fuel transfer conduit and anair-fuel discharge plate, the air-fuel transfer conduit has an upstreamend and a downstream end arranged to lie in spaced-apart relationopposite the upstream end and the air-fuel transfer conduit is coupledto an air-fuel mixing chamber at the upstream end and to the air-fueldischarge plate at the downstream end.
 18. The air-fuel burner of claim17, wherein the first nozzle exit is defined by a series of air-fueldischarge slots formed in the air-fuel transfer conduit and arranged tolie in circumferentially spaced-apart relation to one another around acircumference of the air-fuel transfer conduit.
 19. The air-fuel burnerof claim 17, wherein the second nozzle exit is defined by a band ofperforations formed in the air-fuel transfer conduit and arranged to liecircumferentially around the circumference of the air-fuel transferconduit.
 20. The air-fuel burner of claim 17, wherein the third nozzleexit is defined by a series of staged air-fuel discharge aperturesformed in the air-fuel discharge plate and arranged to extend in apattern between a center of the air-fuel discharge plate and a perimeteredge of the air-fuel discharge plate to cause the attached third flame,when ignited, to extend between the center and the perimeter edge tomaintain ignition of the detached second flame.